In connection with underlayment for beds of ballast, reference may be had to my earlier patents: U.S. Pat. Nos. 4,775,103 issued Oct. 4, 1988; 4,720,043 issued Jan. 19, 1988; 4,696,429 issued Sept. 29, 1987; 4,577,801 issued Mar. 25, 1986 and 4,527,736 issued July 9, 1985.
In a prior mat construction of the above described type, the upper layer mat is formed by a plastically deformable material, for example a bitumen layer (see German Utility Model DE-GM 8013779). In this system, the stones of the ballast bed can penetrate to various degrees into the bitumen layer so that the ballast bed is stabilized against slippage relative to the underlayment. This underlayment, however, has the drawback that sharp-edged ballast stones readily penetrate through the bitumen layer into the elastomeric layer underlying same and cause deterioration of the mat so that, after a time, the entire mat is granulated and no longer has sound-damping properties.
Another mat used as an underlayment for a ballast bed is provided on its upper surface with a sheet metal layer intended to prevent penetration of the ballast into the mat. A mat of this type is described in German Open Application DE-OS 3,121,946. This mat does indeed provide an especially high degree of sound-damping, but has low flexibility so that its emplacement poses a problem. In practice, the mat can only be laid down in relatively short lengths.
In the underlayment described in German Open Application DE-OS 3 425 647 (U.S. Pat. No. 4,696,429), the layer overlying the elastomeric body of the mat is a woven fabric while in German Open Application DE-OS 3 506 505, this layer is a nonwoven fleece.
These mats also provide high degrees of sound damping or, stated otherwise, reduced sound transmission through the solid body of the underlayment. (i.e. noise damping) However, they also require multiple layers to form the upper surface. For example, when a woven fabric is used for the upper part of the mat, two or more layers of fabric are required.
The layers contribute tensile strength to the mats only in the directions in which they extend so that with two layers, for example, tensile strength is provided only in two directions at a right angle to one another. The modulus of the elasticity can also be controlled only in two directions when two woven fabric layers are provided in the manner described.
In general, therefore, I have found that in spite of the high degree of sound-damping which can be obtained utilizing these earlier systems, the systems themselves require improvement.